1. Field of the Invention
The invention relates to a method for the purification of exhaust air containing impurities which are biologically degradable and are particularly difficultly soluble in water. The method includes conducting the exhaust air through an absorption tower in which the impurities are absorbed by a washing liquid containing activated sludge and are subsequently biologically degraded in an activated sludge tank by the microorganisms of the activated sludge.
2. Description of the Prior Art
From the magazine "Staub-Reinhaltung der Luft" [Dust Purification of Air] (1979), pages 308-314, a method for the purification of exhaust air containing biologically degradable impurities is known in which the exhaust air containing the impurities is conducted through an absorption tower filled with packing bodies. An activated sludge washing liquid is conducted through the absorption tower in a counterflow to the exhaust air. The activated sludge washing liquid is pumped to circulate from an activated sludge tank. When the activated sludge washing liquid contacts the exhaust air, the impurities contained in the exhaust air are absorbed by the activated sludge washing liquid. The absorption capacity of the activated sludge washing liquid depends on the solubility of the impurities in the washing liquids and the regeneration thereof. The absorbed impurities are biologically degraded in the activated sludge tank by species-specific microorganisms. The regenerated activated sludge washing liquid is then again capable of absorbing impurities from the exhaust air.
The efficiency of an absorption tower or of a biological washer relative to the purification performance for the individual impurities is thus dependent upon the absorption capacity of the activated sludge washing liquid and the latter, in turn, is dependent upon the rate of degradation of the individual impurities in the activated sludge by the appropriate microorganisms. If the supply of impurities to the liquid phase or the activated sludge washing liquid is greater than the corresponding rate of degradation of the respective impurities in the activated sludge tank, inevitably an enrichment of impurities in the activated sludge washing liquid occurs. With increasing enrichment of impurities, the partial vapor pressure of the impurities above the liquid phase approaches that of the gaseous phase. Thus, the obtainable absorption efficiency of the impurities is limited. Variations of the concentration of the impurities may result in that already absorbed impurities are again set free. This is particularly the case when the exhaust air flow contains impurities which are difficultly soluble in water. This known method for the purification of exhaust air containing biologically degradable impurities does not have a buffer capacity for handling peak loads. The absorption capacity of the activated sludge washing liquid is very low for materials which are difficultly soluble in water. For example, in the case of a concentration of 200 mg/m.sup.3 toluene in the washer inlet, the maximum absorption capacity is 1 mg/l in the washing water so that the minimum amount of washing water for obtaining a significant efficiency is 0.2 to 0.4 m.sup.3 washing water per m.sup.3 exhaust air. This cannot be realized for economical reasons.
German Offenlegungsschrift No. 32 27 375 discloses a method for the purification of exhaust air containing biologically degradable impurities in which pulverized activated carbon is added to the activated sludge washing liquid for increasing the absorption efficiency and for providing a buffer capacity for handling peak loads. The impurities which are difficultly soluble in water are initially adsorbed by the activated carbon and are subsequently degraded by the microorganisms of the activated sludge, i.e., the activated carbon is biologically regenerated. This method has the practical disadvantage that the addition of large amounts of activated carbon causes the solids content of the activated sludge washing liquid to increase and, thus, the danger of clogging of the washing system increases. Particularly when very high-boiling, i.e., very well absorbable impurities are present, these impurities are practically irreversibly bound to the activated carbon over a period of time and then block the activated carbons for the actual low-boiling impurities to be removed from the exhaust air. Moreover, in this method, the material transfer of the impurities to be absorbed to the activated carbon does not take place directly, but rather from the exhaust air through the activated sludge washing liquid to the activated carbon. The last step in the use of conventional absorption towers with packing bodies determines the velocity of the process and is very slow, so that, for a successful use of this method, material exchange apparatus having very high turbulence of the washing water must be used. However, these are only washers which have a very high energy consumption, for example, Venturi washers, washers with perforated bottoms, bubbling tray washers, gas bubble washers.
It is the object of the invention to provide a method for the purification of exhaust air containing impurities which are biologically degradable and particularly difficultly soluble in water, wherein, when utilizing conventional washers or absorption towers having a low energy consumption, the danger of clogging of the washing system is essentially excluded, the obtainable absorption efficiency is increased and a buffer capacity for handling peak loads exists.